1. Field of the Invention
The present invention relates to prestack migration of seismic data. More particularly, the present invention relates to prestack migration of seismic data comprising an ensemble of seismic traces recorded in association with a single shot.
2. Description of the Related Art
Claerbout's imaging principle, that "reflectors exist in the earth at places where the onset of the downgoing wave is time-coincident with an upcoming wave," is the foundation of all extant methods of shot-record migration. Claerbout, J. F., Imaging the Earth's Interior, BLACKWELL SCIENTIFIC PUBLICATIONS, 377 (1985). Claerbout differentiates this idea from the "survey-sinking" idea that employs downward continuation of both common-source and common-receiver records; but other authors argue that the two approaches are equivalent. In any case, Claerbout's idea defines shot-record depth migration and seeks to produce a subsurface image directly from each shot record. In the recent literature, the term "shot-record migration" is synonymous with shot-record depth migration. The recent literature, however, does not recognize the possibility of shot-record migration without concurrent imaging. Extant methods do not produce shot records as outputs, but rather collections of zero-offset traces.
Researchers specializing in the field of geophysics express considerable confidence in the potential efficacy of their own methods. Many would endorse the claim of Al-Yahya that ". . . (the conventional processing stream) should be replaced by the more. theoretically accurate prestack depth migration ...." Al-Yahya, K., Velocity Analysis by Iterative Profile Migration, GEOPHYSICS, 54, 718-729 (1989). At the same time, many authors frankly admit the practical problems encountered in applying their own approaches to routine data processing, and they describe the cost of the individual methods as an impediment. The chief difficulty reported in attempts to apply shot-record depth migration is that the content and quality of the images produced are extremely sensitive to the assumed velocity field. Closely related to the velocity sensitivity is that these solution methods produce results that preclude further processing by other standard techniques. For example, the prior art methods uniformly modify the echo pattern characteristic of a horizontal reflector during processing. Further, these existing methods can be implemented only by digital computers.
The impression created by authors referred to in the immediately preceding paragraph is that prestack depth migration requires or provides a criterion for defining a very detailed picture of subsurface velocities. However, one author (Versteeg, R. J., Sensitivity of Prestack Depth Migration to the Velocity Model, GEOPHYSICS, 58, 873-882 (1993)) presents evidence that if the exact velocity field is known, a rather low-resolution rendering of that field is sufficient to recover an acceptable image, even where there is small-scale velocity variation.
The question may be posed whether the apparent hypersensitivity of shot-record depth migration to velocity is an artifact of the method itself. It seems clear that obtaining a subsurface image directly from a shot record is not a straightforward task. A possible alternative to shot-record depth migration is shot-record time migration. An inference of the existence of shot-record time migration comes from the fact that shot-record DMO exists. For example, if DMO is considered as a zero-offset demigration (i.e., diffraction or modeling) of a prestack migration result, then zero-offset migration of a DMO shot record, followed by reverse NMO, should indicate what a time-migrated shot record looks like in contrast to depth migration, time migration seeks the comparatively modest goal of determining the lateral location of the origin of a seismic echo, while leaving the depth of the object undetermined. The application of other well-known methods for estimating depth may be employed as appropriate or required after use of time migration.
Based upon the foregoing, those skilled in the art should fully understand and appreciate that prior art depth-oriented methods for migrating seismic data share several drawbacks: they demand especially precise prior knowledge of subsurface seismic velocities; they produce results in a form that precludes additional desirable processing by many standard techniques; and they can be implemented only by digital computer. It is a shortcoming and deficiency of the prior art that the seismic exploration industry lacks a method of shot migration that allows the use of conventional velocity analysis, common-midpoint stacking, and other conventional procedures as post-processing techniques. It is also a shortcoming and deficiency of the prior art that the seismic exploration industry lacks a method of prestack migration of seismic data that can be performed by hand as well as by digital computer. Yet another shortcoming and deficiency of the prior art is the lack of a method of prestack migration-of seismic data wherein the echo pattern characteristic of a horizontal reflection, as observed in a seismic shot record, passes through the process without modification.